Semiconductor device and fabricating method thereof

ABSTRACT

A semiconductor device includes a fin structure extending along a first direction, a channel layer wrapping around a top surface and opposite sidewalls of the fin structure, a gate stack extending across the channel layer along a second direction perpendicular to the first direction, and a spacer on a top surface of the channel layer and a sidewall of the gate stack when viewed in a cross section taken along the first direction. The channel layer includes a two-dimensional material. The gate stack includes a ferroelectric layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No. 15/904,699, filed Feb. 26, 2018, now U.S. Pat. No. 10,490,631, issued Nov. 26, 2019, which claims priority to U.S. Provisional Application Ser. No. 62/590,396, filed Nov. 24, 2017, which are herein incorporated by reference in their entirety.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications such as, for example, personal computers, cell phones and other electronic equipment. Transistors are circuit components or elements that are often formed on semiconductor devices. With the progress of transistor process technology, the dimension of transistors continues to shrink and therefore the number of transistors per unit area of an integrated circuit has increased accordingly.

Methods of fabrication of field effect transistor (FET) devices may be incapable of controlling the layer thickness and interface composition with atomic precision. For example, the physical limitation (i.e. 60 mV/decade) restricts the subthreshold swing, making complementary metal-oxide-semiconductor (CMOS) voltage and transistor power dissipation not be downscaled arbitrarily.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart of a method of fabricating a semiconductor device, in accordance with some embodiments.

FIGS. 2A to 20D illustrate a method of fabricating a semiconductor device at various stages in accordance with some embodiments.

FIG. 21 is an enlarged view of area M of FIG. 20D.

FIG. 22 is an enlarged view of area N of FIG. 20C.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. In addition, the term “made of” may mean either “comprising” or “consisting of.”

FIG. 1 is an exemplarily process flow chart 100 for fabricating a semiconductor device, according to one or more embodiments of the present disclosure. The method includes operation 101 in which a fin structure is formed on a semiconductor substrate. The method continues with operation 103 in which a passivation layer is formed on the fin structure. Subsequently, operation 105, a channel layer is formed on the passivation layer. The method continues with operation 107 in which a dielectric layer is formed on the channel layer. The method continues with operation 109 in which a cap layer is formed on the dielectric layer. Next, operation 111, an annealing process is performed. The method continues with operation 113 in which a semiconductor gate is formed on the cap layer. In operation 115 the semiconductor gate is patterned to form a gate stack with the annealed dielectric layer and the annealed cap layer. It is understood that additional operations may be provided before, during, and after the operations shown by FIG. 1, and some of the operations described below can be replaced or eliminated for additional embodiments of the method. The order of the operations/processes may be interchangeable.

FIGS. 2A to 20D illustrate a method of fabricating a semiconductor device at various stages in accordance with some embodiments of the present disclosure. FIGS. 2A through 22 are respectively perspective and cross-sectional views of various intermediary stages in manufacturing of the semiconductor device in accordance with various embodiments. In FIGS. 2A through 22, figures ending with an “A” designation are illustrated a perspective view of the semiconductor device, figures with a B designation are illustrated along a similar cross-section B-B′, figures ending with a “C” designation are illustrated along a similar cross-section C-C′, and figures ending with a “D” designation are illustrated along a similar cross-section D-D′. In some embodiments, the semiconductor device shown in FIGS. 2A to 20D may be intermediate devices fabricated during processing of an integrated circuit (IC), or a portion thereof, that may include static random access memory (SRAM), logic circuits, passive components, such as resistors, capacitors, and inductors, and/or active components, such as p-type field effect transistors (PFETs), n-type FETs (NFETs), multi-gate FETs, metal-oxide semiconductor field effect transistors (MOSFETs), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof.

Referring to FIGS. 2A to 2C. A semiconductor substrate 201 is provided. In some embodiments, the semiconductor substrate 201 may include an elementary semiconductor including silicon or germanium in a crystalline, a polycrystalline, or an amorphous structure. In some embodiments, the semiconductor substrate 201 may include an alloy semiconductor including SiGe, GeSn, SiGeSn, III-V alloys and/or any other suitable material, or combinations thereof. In some embodiments, the semiconductor substrate 201 includes a single layer having a thickness in a range from about 30 to about 100 nm. In some embodiments, the semiconductor substrate 201 includes a multilayered structure, and at least one of the layers has a thickness in a range from about 3 to about 10 nm. In some embodiments, the semiconductor substrate 201 may be strain relaxed buffer (SRB) layer.

In FIGS. 3A to 3C, a fin structure 213 is formed over the semiconductor substrate 201 (operation 101 of FIG. 1). Specifically, a patterned mask layer 205 (may be a hard mask layer) is formed over the top surface of the substrate 201. The mask layer 205 maintains the integrity of the patterns during etching of trenches 202 formed in the substrate 201. In some embodiments, the mask layer 205 is used as a planarization stop layer during the removal of excess dielectric layer that fills the trenches 202. In some embodiments, the mask layer 205 includes nitride. For example, the mask layer 205 is made of silicon nitride (SiN). However, other materials, such as SiON, silicon carbide, or a combination thereof, may also be used. The mask layer 205 may be formed by a process such as chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or low pressure chemical vapor deposition (LPCVD). Alternatively, the mask layer 205 may be made of a silicon oxide and then converted to SiN by nitridation. The terms “trench” used herein may broadly cover any topographic features such as “opening,” “holes,” “channels,” “recesses,” “plugs,” etc., that are suitable for isolation or providing a conductive path which extend horizontally and/or vertically within the structure.

In some embodiments, a protective layer 203 is formed over the top surface of the substrate 201 and between the mask layer 205 and the substrate 201. The protective layer 203 protects the top surface from direct contact with the mask layer 205. For example, for a portion of the substrate 201 next to the trench 202, the protective layer 203 can protect regions formed in the portion of the substrate 201. In some embodiments, the protective layer 203 is made of a thermal oxide. Once formed, the mask layer 205 and the protective layer 203 are patterned through suitable photolithographic and etching processes to form openings over the top surface of the substrate 201 for the trenches 202.

The exposed portions of the substrate 201 through the openings of the mask layer 205 and the protective layer 203 are removed by an etching process, such as reactive ion etching (RIE), in order to form the trenches 202 in the substrate 201. An aspect ratio, the depth of the trench 202 (sometimes referred to herein as trench height) divided by the width of the trench 202, can be greater than about 7. In some other embodiments, the aspect ratio may even be greater than about 8, although it may also be lower than about 7, or between 7 and 8. One skilled in the art will realize, however, that the dimensions and values recited throughout the descriptions are merely examples, and may be changed to suit different scales of semiconductor devices.

In some embodiments, the trenches 202 are configured to separate adjacent two fin structure 213 formed on the substrate 201. In other words, one of the fin structures 213 is disposed between adjacent two of the trenches 202. In some embodiments, the fin structures 213 are separated from each other by a pitch P₁ in a range from about 12 to about 50 nm, and at least one of the fin structures 213 has a width W₁ in a range from about 4 to about 8 nm.

A bottom liner 207 is conformally formed over the mask layer 205 and in the trenches 202, and a top liner 208 is conformally formed over the bottom liner 207. The bottom liner 207 may be made of silicon nitride (SiN). However, other materials, such as SiON, silicon carbide, or a combination thereof, may also be used. The top liner 208 may be made of silicon oxide or other suitable materials.

A dielectric material is formed over the top liner 208 and overfills the trenches 202 to form a dielectric structure 209. The dielectric structure 209 can be formed by using a spin on dielectric (SOD) formation process, or by depositing a flowable dielectric by a chemical vapor deposition (CVD) process, such as radical-component CVD. In some embodiments, the dielectric structure 209 can be formed using, for example, flowable chemical vapor deposition (FCVD), and performing a chemical mechanical polish (CMP) to level the top surface of the dielectric material with the top surface of the bottom liner 207. Therefore, the bottom liner 207 is exposed from the dielectric structure 209.

Turning now to FIGS. 4A to 4C, the fin structures 213 are exposed by recessing the dielectric structure 209, the bottom liner 207, and the top liner 208. Specifically, in FIG. 4A, the mask layers 205 and the protection layers 203 of FIGS. 3A to 3C are removed, and then the dielectric structure 209, the bottom liner 207, and the top liner 208 are recessed such that upper regions of the fin structures 213 are exposed. That is, sidewalls 214 and top surfaces of the upper regions of the fin structures 213 are exposed. The dielectric structure 209, the bottom liner 207, and the top liner 208 may be recessed by suitable methods, such as an etching process. As shown in FIG. 4B, the upper regions of the fin structures 213 are not covered by the bottom liner 207 and the top liner 208. The dielectric structure 209 is disposed between lower regions of the fin structures 213 such that the upper regions may be exposed for the subsequent processes. In some embodiments, at least one of the upper regions of the fin structures 213 has a height H₁ in a range from about 30 to about 100 nm. In FIG. 4C, the bottom liner 207 and the top liner 208 above the top surface of the fin structure 213 has been removed at this stage.

Referring to FIGS. 5A to 5C, passivation layers 215 are respectively formed on the fin structures 213 (operation 103 of FIG. 1). Specifically, a passivation material is conformally deposited over the dielectric structures 209 and the fin structures 213. Subsequently, the passivation material is partially removed to form the passivation layers 215 respectively cover the upper portions of the fin structures 213. That is, after partially removing the passivation material, the passivation layers 215 are formed independently on the fin structures 213 such that the dielectric structure 209 is not covered by the passivation layers 215. The passivation layer 215 extends along the sidewalls 214 and the top surface of the fin structures 213 and is in contact with the bottom liner 207 in some embodiments. As shown in FIG. 5C, the passivation layer 215 is formed on the top surface of the fin structure 213. In some embodiments, the passivation layer 215 is made of an insulating material, such as a two-dimensional (2D) material (e.g., boron nitride or other suitable 2D materials). 2D materials are crystalline materials made from a single layer of atoms. Example of 2D materials may include, but are not limited to transition metal dichalcogenides (TMDs) having the formula MX₂, in which M may be Mo or W, and X may be S, Se or Te. Other examples of 2D materials may include, but are not limited to black phosphorus (BP), metal carbides (e.g., Ti₂C and Ti₂CF₂) and the like. In some embodiments, the passivation layer 215 having boron nitride is deposited using plasma-enhanced atomic layer deposition (PE-ALD), such as Tris (ethylmethylamino) borane (TEMAB, C₉H₂₄BN₃) and/or NH₃/N₂ plasma, at a temperature ranging from about 250° C. to about 300° C., and has a thickness in a range from about 1 to about 5 nm. The boron nitride used herein as the passivation layer 215 also has an insulating property. In some embodiments, the passivation layer 215 is deposited using chemical vapor deposition (CVD).

In some embodiments, the passivation layer 215 may be made of boron nitride. The passivation layer 215 having boron nitride can help prevent additional scattering of the subsequent material to be formed on associated with the low surface phonon energy and large trap density, thereby improving both of device properties and uniformity across the wafer. The passivation layer 215 having boron nitride, more specifically, provides a smooth surface that is relatively free of dangling bonds and charge traps, and the different on-site energies of the boron and nitrogen atoms provide a good match of the lattice constant with other 2D materials (e.g., merely about 1.7% mismatch with graphite), and a large electrical band gap (e.g., about 6 eV).

Referring to FIGS. 6A to 6C, channel layers 217 are respectively formed on the passivation layer 215 and over the fin structures 213 (operation 105 of FIG. 1). Specifically, a channel material is conformally deposited over the isolation structures 207 and the passivation layers 215. Subsequently, the channel material is partially removed to form the channel layers 217 respectively on the passivation layers 215. That is, after partially removing the channel material, the channel layers 217 are formed independently over the passivation layers 215 such that the dielectric structure 209 is not covered by the channel layers 217. The channel layer 217 over the passivation layer 215 extends along sidewalls and a top surface of the passivation layer 215.

As shown in FIG. 6C, the channel layer 217 is formed on the passivation layer 215 over the fin structure 213. In some embodiments, the channel layer 217 is made of a 2D conductive material, such as TMDs, BP, metal carbides, or other suitable materials. In some embodiments, the passivation layer 215 and the channel layer 217 are made of 2D materials different from each other. For example, the channel layer 217 (e.g. MoS₂, conductive) is deposited on the passivation layer 215 (e.g. BN, insulating) using Atmospheric Pressure CVD (AP-CVD). During the AP-CVD using argon (Ar) as the carrier gas, sulfur (S) is introduced by heating to about 180° C., molybdenum trioxide (MoO₃) is introduced by heating to about 300° C. and the wafer is introduced by heating to about 800° C. In some embodiments, the channel layer 217 has a thickness in a range from about 1 to about 5 nm. In some embodiments, the channel layer 217 can be formed using other suitable deposition method such as, for example, ALD, or PE-CVD. In some embodiments, the channel layer 217 may be monolayer that has a thickness smaller than 1 nm. In some embodiments, the channel layer 217 may include multiple layers that have a combined thickness smaller than 1 nm.

In some embodiments, due to lack of dangling bonds (which serve as nucleation sites for the materials to be uniformly deposited using ALD or the like) on the 2D material's basal plane, an activated surface is desired. Therefore, a surface treatment is performed on the channel layer 217 having a 2D material before the formation of the gate dielectric layer. In some embodiments, the surface treatment may be, for example, oxygen plasma treatment, UV-O₃ treatment, utilization of metal oxide seed layer(s), or combinations thereof. For example, at least one metal oxide layer 218 is formed on the channel layer 217 by performing the metal oxide seed layer(s) utilization.

It is noted that the passivation layers 215 and the channel layers 217 are separately patterned in FIGS. 5A to 6C. That is, the channel layers 217 are patterned after than passivation layers 215 are patterned. In some other embodiments, however, passivation materials and channel materials can be sequentially formed, and the passivation materials and the channel materials are then patterned in the same process to form the passivation layers 215 and the channel layers 217.

Referring to FIGS. 7A to 7C, a dielectric layer 219 is formed on the channel layer 217 (operation 107 of FIG. 1), specifically, on a surface of the channel layer 217 or the metal oxide layer 218 after the surface treatment. A precursor material is introduced to the channel layers 217. The precursor material is doped with impurities to form the dielectric layer 219. As shown in 7A and 7B, the dielectric layer 219 continuously covers the isolation structure 207 and sidewalls and top surfaces of the channel layer 217 across the fin structures 213. Since the surface treatment described above is performed on the channel layer 217, the dielectric layer 219 can be uniformly formed over the channel layer 217. In some embodiments, the dielectric layer 219 is in-situ doped with semiconductor material (e.g. Si) or metal elements (e.g. Zr, Al, La, Y, Gd, Sr) during ALD growth on the channel layer 217 after the surface treatment. For example, in-situ doping for Zr-doped HfO₂ may involves a reaction between (HfCl₄+H₂O) and (ZrCl₄+H₂O) using nanolaminate ALD per cycle at about 300° C. In some embodiments, the dielectric layer 219 having HfO₂ is doped with Si ranging from about 2 to about 6 molar percentage (mol %). In some other embodiments, the dielectric layer 219 having HfO₂ is doped with Zr in a ratio of Hf:Zr=1:1. In yet some other embodiments, the dielectric layer 219 having HfO₂ is doped with Al ranging from about 7 to 11 mol %. As shown in FIG. 7C, the dielectric layer 219 is formed on the top surface of the channel layer 217 over the passivation layer 215. In some embodiments, the dielectric layer 219 has a thickness in a range about 1 to about 5 nm. The crystal structure of the dielectric layer 219 at this stage is amorphous.

Referring to FIGS. 8A to 8C, a cap layer 221 is conformally formed over the dielectric layer 219 (operation 109 of FIG. 1). In FIGS. 8A and 8B, the cap layer 221 continuously covers along bottom surfaces, sidewalls and top surfaces of the dielectric layer 219 across the fin structures 213. As shown in FIG. 8C, the cap layer 221 is formed on the top surface of the dielectric layer 219. In some embodiments, the cap layer 221 may be deposited by ALD. The cap layer 221 may include a single layer or a multilayered structure and at least one of the layers has a thickness in a range from about 1 to about 5 nm. For example, the cap layer 221 includes a capping metal layer on the dielectric layer 219, a barrier metal layer on the capping metal layer, and a work function metal layer on the barrier metal layer. In some embodiments, the capping metal layer is made of TiN or TSN, the barrier metal layer is made of TaN, and the work function metal can be P work function metal (e.g., TiN) or N work function metal (e.g., TiAl or TaAl). In some other embodiments, the cap layer 221 may further include a glue layer made of TiN.

Referring to FIGS. 9A to 9C, an annealing process 10 is performed (operation 112 of FIG. 1) on the dielectric layer 219 (see FIGS. 8A to 8C) and the cap layer 221. As described above, the crystal structure of the dielectric layer 219 is amorphous as shown in FIGS. 7A to 7C. Transition from the amorphous phase of the dielectric layer 219 to the orthorhombic phase of a ferroelectric layer 219′ is desired for good ferroelectric properties. In the annealing process 10, a suitable temperature provides the driving force for crystalline transition of the dielectric layer 219 from the amorphous phase to the high temperature tetragonal phase (a=b≠c, α=β=γ=90°), and then the cap layer 221 provides the mechanical stress for the dielectric layer 219 transition from the tetragonal phase to the high pressure ferroelectric orthorhombic phase (a≠b≠c, α=β=γ=90°) during cooling. During the cooling stage, the cap layer 221 with capping metal layer that in contact with the dielectric layer 219 may help increase intrinsic polarization generated by 4 oxygen ions displacement. After the annealing process 10, the dielectric layer 219 is transformed to the ferroelectric layer 219′. In some embodiments, the annealing process 10 is performed at a temperature about 600° C. to about 1000° C. under atmospheric pressure in N₂ environment. In some embodiments, the annealing process 10 is a spike anneal for a duration of about 30 seconds. In addition, while the annealing process 10 discussed herein is carried out before metallization (i.e., post-cap anneal, PCA), it is contemplated that the annealing process 10 may be carried out after metallization (i.e., post-metal anneal, PMA).

Referring to FIGS. 10A to 10D, a semiconductor gate is formed over the cap layer 221, and at least one gate stack 227 including the semiconductor gate layer 223, the cap layer 221, and the ferroelectric layer 219′ is formed on the fin structures 213 (operation 115 and operation 118 of FIG. 1). A semiconductor material layer, such as a polysilicon layer, is deposited over the cap layer 221. A hardmask material layer is deposited over the semiconductor material layer. Next, for forming the gate stacks 227, a patterning process, including such as a photolithographic process, is performed on the semiconductor material layer, the cap layer 221, and the ferroelectric layer 219′. The photolithography process includes coating the hardmask material layer with a photoresist, selectively exposing the photoresist according to a desired pattern, developing the photoresist, and using the patterned photoresist as an etch mask. An etching process is then performed to the hardmask material layer upon which the patterned photoresist is utilized, thereby forming a patterned hardmask layer 225. Subsequently, an etching process is performed to the semiconductor material layer upon which the patterned hardmask layer 225 is utilized, thereby forming a patterned semiconductor gate layer 223. Then, portions of the cap layer 221 and the ferroelectric layer 219′ are removed by, for example, etching, to define the gate stacks 227. At least one of the gate stacks 227 includes the ferroelectric layer 219′, the cap layer 221 and the semiconductor gate layer 223. As shown in FIG. 10B, a portion of the dielectric structure 209 between two fin structures 213 is exposed after the semiconductor gate patterning process. The hardmask layer 225, the semiconductor gate layer 223, the cap layer 221 and the ferroelectric layer 219′ has been etched to expose the portion of the dielectric structure 209. In some embodiments, a portion of the dielectric structure 209 between the adjacent fin structures 213 is exposed after the polysilicon gate patterning process. In some embodiments, fin structures 213 independently surrounded with the hardmask layer 225, the gate stack 227, the channel layer 217 and the passivation layer 215 are separated from each other by a space created from the removed portion of the ferroelectric layer 219′, the cap layer 221, the semiconductor gate layer 223 and the hardmask layer 225.

In FIG. 10C, the channel layer 217 between the gate stacks 227 on each of the fin structures 213 are exposed after the semiconductor gate patterning process. Also, the dielectric structure 209 between the fin structures 213 is exposed. In FIG. 10D, the channel layer 217 between the gate stacks 227 are exposed after the semiconductor gate patterning process. The hardmask layer 225, the semiconductor gate layer 223, the cap layer 221 and the ferroelectric layer 219′ are removed between the gate stacks 227 respectively on the fin structures 213, and thus the gate stacks 227 are separated from each other by a space.

It is understood that four fin structures 213 and twelve gate stacks 227 are shown for illustration purposes. The fin structures and gate structures may be in any numbers depending on the application. In addition, while the gate stacks 227 discussed herein are fabricated using a gate-first process, it is contemplated that the gate stacks 227 may be fabricated using a gate-last process.

Referring to FIGS. 11A to 11D, a spacer layer 229 a is formed over the gate stacks 227 and the dielectric structure 209. The method of forming the spacer layer 229 a includes blanket forming a spacer material layer on the structure shown in FIG. 10A by, for example, performing an ALD process. In some embodiments, the spacer layer 229 a may include a single layer or multilayer structure made of different dielectric materials. In some embodiments, the spacer layer 229 a is made of SiN.

Turning now to FIGS. 12A to 12D, the spacer layer 229 a of FIGS. 11A to 11D is partially removed to expose the passivation layer 215 (as shown in FIGS. 12C and 12D). For example, an etching process is performed to the spacer layer 229 a to form first spacers 229 b. In FIGS. 12A and 12B, a top surface of the hardmask layer 225 is exposed after the etching process. The top surface of the hardmask layer 225 is substantially coplanar with a top surface of the first spacers 229 b. The gate stacks 227 are electrically separated from each other by the first spacers 229 b. In FIG. 12C, sidewalls of the spacer layer 229 a between the fin structures 213 may be pullback-etched. The first spacers 229 b remain at corners between the dielectric structure 209 and the channel layer 217 on the opposing sides of the upper region of the fin structures 213 after the etching. For the subsequent process to be discussed in FIGS. 13A to 13D, a portion of the channel layer 217 over the fin structure 213 is also removed to expose the passivation layer 215 below. FIG. 12D further illustrates the passivation layer 215 is exposed between the gate stacks 227 flanked by the first spacers 229 b. Sidewalls 235 of the channel layer 217 over the passivation layer 215 between the gate stacks 227 are also exposed after the etching process. The sidewall 235 of the channel layer 217 is substantially coplanar with a sidewall of the first spacer 229 b. In some embodiments, the etching process may include dry etching, wet etching, and/or other suitable etching methods.

Referring to FIGS. 13A to 13D, a source/drain metal layer 233 a is blanket formed over the first spacers 229 b. The source/drain metal layer 233 a is formed over the fin structures 213 between the gate stacks 227, and directly covers the exposed passivation layer 215 where the portion of the channel layer 217 is removed in FIGS. 12A to 12D. In FIG. 13B, the source/drain metal layer 233 a is disposed on the first spacers 229 b and the hardmask layer 225. In FIG. 13C, the source/drain metal layer 233 a continuously extends along the Y-axis direction covering the isolation structure 207, the first spacers 229 b, the channel layer 217 and the passivation layer 215 across the fin structures 213. That is, the source/drain metal layer 233 a is in contact with the channel layer 217 uncovered by the gate stacks 227 (see FIG. 13A). FIG. 13D further illustrates the source/drain metal layer 233 a also continuously extends along the X-axis direction across the gate stacks 227. The source/drain metal layer 233 a is also in contact with the portion of the channel layer 217 covered by the gate stacks 227. The source/drain metal layer 233 a is further in contact with the exposed passivation layer 215. In some embodiments, the source/drain metal layer 233 a can be deposited using, for example, ALD, CVD, PVD, or other suitable deposition methods. In some embodiments, the source/drain metal layer 233 a has a thickness in a range from about 1 to about 5 nm. In some embodiment, the source/drain metal layer 233 a includes Au, Ti, Pt, Al, Pt, Ni, Sc, Ir, or combinations thereof. Different types of the channel layer 217 that includes the 2D material may be suitable for correspondingly different contact metals, depending on the applications. For example, Ti or Sc is utilized as the source/drain metal layer 233 a in N-type field-effect transistors (FET). In some embodiments, Ni or Pt is utilized as the source/drain metal layer 233 a in P-type FET. The process concepts described above can be integrated into the present Fin FET fabrication process, and can be implemented in a number of technology nodes.

Referring to FIGS. 14A to 14D, a first interlayer dielectric (ILD) layer 239 is formed over the source/drain metal layer 233 a. In FIG. 14A, the first ILD layer 239 is disposed over and in contact with the source/drain metal layer 233 a. As shown in FIGS. 14C and 14D, the first ILD layer 239 adheres to the source/drain metal layer 233 a across the fin structures 213 and the gate stacks 227. Recesses between the upper regions of the fin structures 213 and recesses between the gate stacks 227 over the fin structures 213 may be filled with the first ILD layer 239. In some embodiments, the first ILD layer 239 may be an oxide such as, for example, silicon dioxide (SiO₂), silicon nitride (SiN), tetraethoxysilane (TEOS), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), low-k dielectric material, or other suitable dielectric materials.

Referring now to FIGS. 15A to 15D, a planarization process is performed to the first ILD layer 239. For example, a CMP process may be used to remove excessive portions of the first ILD layer 239 and the hardmask layer 225 over the gate stacks 227 to expose the gate stacks 227. The first spacers 229 b and the source/drain metal layer 233 a of FIGS. 14A to 14D have been leveled down to form second spacers 229 c and source/drain layers 233 b, respectively. In some embodiments, the CMP process is performed to remove the hardmask layer 225 from an upper surface of the semiconductor gate layer 223, as shown in FIG. 15B. In FIG. 15C, the first ILD layer 239 interposed between the gate stacks 227 has been leveled down and has a substantially flat upper surface over the source/drain layers 233 b. FIG. 15D further illustrates a profile of the source/drain layers 233 b. After the planarization process, an upper surface of the planarized first ILD layer 239, an upper surface of the source/drain layers 233 b, an upper surface of the second spacers 229 c, and the upper surface of the semiconductor gate layer 223 are substantially coplanar. The source/drain layer 233 b has a U-shaped profile along sidewalls of the second spacers 229 c, the sidewalls 235 of the channel layer 217 and the top surface of the passivation layer 215 such that the sidewalls 235 of the channel layer 217 and the top surface of the passivation layer 215 are in contact with the source/drain layers 233 b, and the planarized first ILD layer 239 is partially wrapped by the source/drain layers 233 b. In some embodiments, the source/drain layers 233 b may be formed with various profiles, such as, for example, when the sidewalls 235 of the channel layer 217 extend horizontally (i.e., in the X-axis direction) along the top surface of the passivation layer 215 beyond the sidewalls of the second spacers 229 c. In some embodiments, at least one of the sidewalls 235 of the channel layer 217 is not coplanar with each of the sidewalls of the second spacers 229 c, that is, the sidewalls 235 of the channel layer 217 protrude beyond the sidewalls of the second spacers 229 c and thus the channel layer 217 may provide more contact area with the source/drain layers 233 b.

Referring now to FIGS. 16A to 16D, a second ILD layer 241 is formed over the structure of FIGS. 15A to 15D (i.e., over the source/drain layers 233 b, the first ILD 239, and the gate stacks 227). The second ILD layer 241 may help electrically insulate the source/drain layers 233 b from the contacts that is for the subsequent desired contact design. In FIG. 16B, the second ILD layer 241 is disposed over the second spacers 229 c and the semiconductor gate layer 223. The second ILD layer 241 has a substantially flat bottom surface coplanar with the upper surface of the second spacers 229 c and the upper surface of the semiconductor gate layer 223 due to aforementioned planarization process. In FIG. 16C, the second ILD layer 241 is formed on and has the substantially flat bottom surface coplanar with a top surface of the first ILD layer 239. In FIG. 16D further illustrates the second ILD layer 241 extends along the X-axis direction across the gate stacks 227. The second ILD layer 241 covers the upper surface of the source/drain layers 233 b, the upper surface of the second spacers 229 c, and the upper surface of the semiconductor gate layer 223. In some embodiments, the second ILD layer 241 is made of an oxide different from the first ILD layer 239.

Referring to FIGS. 17A to 17D, a contact etch stop layer (CESL) 243 is formed on the second ILD layer 241. The CESL 243 can be formed using, for example, plasma enhanced CVD, low pressure CVD, ALD or other suitable techniques. In some embodiments, the CESL 243 includes silicon nitride, silicon oxynitride or other suitable materials. In some embodiments, the CESL 243 may include a material different from the second ILD layer 241.

Referring to FIGS. 18A to 18D, via openings 245 are formed. The via openings 245 may be alternately arranged on the opposing sides of the gate stacks 227. The via openings 245 are formed to expose the source/drain layers 233 b, as illustrated in FIG. 18C. In some embodiments, the via openings 245 has an upper region over the fin structures 213 and a lower region between two adjacent fin structures 213. The upper region of the via openings 245 is wider than the lower region of the via openings 245 such that the via opening 245 has a profile tapered down from a top surface of the CESL 243 toward the isolation structure 207. In some embodiments, the via openings 245 may be formed with various profiles different from each other. In FIG. 18D further illustrates the via openings 245 are formed adjacent to the gate stack 227. The via openings 245 expose the source/drain layers 233 b where the passivation layer 215 and the sidewalls 235 of the channel layer 217 are in contact with the source/drain layers 233 b.

Referring to FIGS. 19A to 19D, a filling metal layer 249 a fills in the via openings 245 over the CESL 243. In some embodiments, before filling the filling metal layer 249 a, a metal glue layer 247 is deposited over the surface of the source/drain layers 233 b. The metal glue layer 247 can improve adhesion between the source/drain layers 233 b and the filling metal layer 249 a. As shown in FIGS. 19C and 19D, the metal glue layer 247 fills a portion of the via openings 245 over the source/drain layers 233 b prior to the filling metal layer 249 a, and the metal glue layer 247 has a top surface over the top surface of the CESL 243. Subsequently, the filling metal layer 249 a fills the remaining portion of the via openings 245 and is formed over the top surface of the metal glue layer 247. As shown in FIGS. 19C and 19D, a relatively large part of the metal glue layer 247 fills in the lower region of the via openings 245 covering the surfaces of the source/drain layers 233 b and thus enhances adhesion between the source/drain layers 233 b and the filling metal layer 249 a. The filling metal layer 249 a is deposited over the metal glue layer 247 and electrically connected to the source/drain layers 233 b. In some embodiments, the metal glue layer 247 may include one or more layers of a material such as, for example, titanium, titanium nitride, titanium tungsten or combinations thereof. In some embodiments, the filling metal layer 249 a may be made of, for example, tungsten (W).

Referring to FIGS. 20A to 20D, a planarization process is performed to the filling metal layer 249 a and the metal glue layer 247 of FIGS. 19A to 19D. Excessive filling metal layer 249 a and the metal glue layer 247 on the top surface of the CESL 243 can be removed via suitable processes such as CMP, thereby forming contact plugs 249 b adhered and electrically connected to the source/drain layers 233 b with the metal glue layer 247.

FIG. 21 is an enlarged view of area M of FIG. 20D and FIG. 22 is an enlarged view of area N of FIG. 20C. As shown in FIG. 21, the semiconductor device includes the fin structure 213, the passivation layer 215, the channel layer 217, and the gate stack 227. The fin structure 213 is extended from the semiconductor substrate 201 (see FIG. 20A). The passivation layer 215 is formed on the fin structure 213. The channel layer 217 is formed on the passivation layer 215. In addition, the sidewalls 235 of the channel layer 217 are on the opposing sides of the gate stack 227 such that the channel layer 217 partially covers the top surface of the passivation layer 215 and partially cover the fin structure 213. Therefore, the source/drain layers 233 b are in direct contact with the channel layer 217 and the passivation layer 215. Moreover, the source/drain layers 233 b are physically and electrically isolated from the fin structure 213. In some embodiments, the passivation layer 215 and the channel layer 217 are made of different 2D materials. For example, the passivation layer 215 is made of boron nitride and the channel layer 217 is made of TMDs, BP or metal carbides.

The gate stack 227 is disposed on the channel layer 217. The gate stack 227 includes the ferroelectric layer 219′, the cap layer 221 and the semiconductor gate layer 223. The ferroelectric layer 219′ is formed over and in contact with the channel layer 217, the cap layer 221 is formed over the ferroelectric layer 219′, and the semiconductor gate layer 223 is formed over the cap layer 221. In addition, the gate stack 227 may be further flanked by a pair of the second spacers 229 c over the channel layer 217. A bottom surface of the second spacer 229 c is substantially coplanar with a bottom surface of the gate stack 227 and the top surface of the channel layer 217. Further, the contact plugs 249 b attach on the source/drain layers 233 b with help of the metal glue layer 247 and thus are electrically connected to the source/drain layers 233 b, as shown in FIGS. 21 and 22. It is understood that the contact design over two fin structures 213 is shown in FIG. 22 for illustration purposes. The metal contact area may be various depending on the application.

The fabrication processes described above integrate ferroelectric dielectrics, 2D materials and the gate-first process into a FinFET transistor. The ferroelectric dielectrics used in negative-capacitance (NC) technology provide a feasible solution to lower V_(DD) significantly and achieve steep subthreshold swing (SS) for low power operation. The 2D materials may be used as materials of channel and/or passivation layer underneath the channel. The gate-first process integrated in the present disclosure offers a low thermal budget (for example, no S/D activation annealing). 2D material is atomically thin, flat, no dangling bond of the surface, and layered structure which is with ease of integration in a junctionless transistor. Compared with bulk silicon and ultra thin body (UTB) FETs, 2D materials can achieve higher voltage amplification in NC-FET structure since they can reach larger capacitance C_(dep) due to limited depletion region width coming from atomically thin structures.

In some embodiments, a semiconductor device comprises a fin structure extending along a first direction, a channel layer wrapping around a top surface and opposite sidewalls of the fin structure, a gate stack extending across the channel layer along a second direction perpendicular to the first direction, and a spacer on a top surface of the channel layer and a sidewall of the gate stack when viewed in a cross section taken along the first direction. The channel layer includes a two-dimensional material. The gate stack includes a ferroelectric layer.

In some embodiments, a semiconductor device comprises a fin structure, a 2D-material passivation layer wrapping around three sides of the fin structure, a 2D-material channel layer wrapping around three sides of the 2D-material passivation layer, and a gate stack wrapping around three sides of the 2D-material channel layer. The gate stack includes a ferroelectric layer.

In some embodiments, a method of fabricating a semiconductor device comprises etching a substrate to form a fin structure, depositing a passivation layer that includes a two-dimensional material on the fin structure, forming a gate stack that includes a ferroelectric layer on the passivation layer, and forming a source/drain metal layer adjacent the gate stack and having a U-shaped cross-section.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device comprising: a fin structure extending along a first direction; a channel layer wrapping around a top surface and opposite sidewalls of the fin structure, wherein the channel layer includes a two-dimensional material; a gate stack extending across the channel layer along a second direction perpendicular to the first direction, wherein the gate stack includes a ferroelectric layer; and a spacer on a top surface of the channel layer and a sidewall of the gate stack when viewed in a cross section taken along the first direction.
 2. The semiconductor device of claim 1, wherein the gate stack further includes a cap layer on a top surface of the ferroelectric layer and a gate layer on a top surface of the cap layer.
 3. The semiconductor device of claim 1, further comprising a passivation layer sandwiched between the fin structure and the channel layer.
 4. The semiconductor device of claim 1, further comprising a passivation layer sandwiched between the fin structure and the spacer.
 5. The semiconductor device of claim 1, further comprising a source/drain metal layer on a sidewall of the spacer when viewed in the cross section take along the first direction.
 6. The semiconductor device of claim 1, further comprising a source/drain metal layer on a sidewall of the channel layer when viewed in a cross section taken along the second direction.
 7. The semiconductor device of claim 1, further comprising a source/drain metal layer that has a top surface substantially coplanar with a top surface of the gate stack.
 8. The semiconductor device of claim 1, further comprising: a source/drain metal layer on the fin structure; a metal glue layer on a sidewall of the source/drain metal layer; and a contact plug on a top surface of the metal glue layer.
 9. The semiconductor device of claim 1, further comprising: a source/drain metal layer; and a passivation layer sandwiched between the fin structure and the source/drain metal layer.
 10. A semiconductor device comprising: a fin structure; a 2D-material passivation layer wrapping around three sides of the fin structure; a 2D-material channel layer wrapping around three sides of the 2D-material passivation layer; and a gate stack wrapping around three sides of the 2D-material channel layer, wherein the gate stack includes a ferroelectric layer.
 11. The semiconductor device of claim 10, further comprising a spacer alongside the gate stack and a source/drain metal layer adjacent the spacer.
 12. The semiconductor device of claim 10, further comprising a source/drain metal layer having a top surface substantially coplanar with a top surface of the gate stack.
 13. The semiconductor device of claim 10, further comprising: a source/drain metal layer on a top surface of the 2D-material passivation layer; a metal glue layer on a sidewall of the source/drain metal layer; and a contact plug on a top surface of the metal glue layer.
 14. The semiconductor device of claim 10, further comprising: a source/drain metal layer, wherein the 2D-material passivation layer is between the fin structure and the source/drain metal layer.
 15. The semiconductor device of claim 10, wherein the gate stack further includes a cap layer on a top surface of the ferroelectric layer and a gate layer on a top surface of the cap layer.
 16. A semiconductor device comprising: a fin structure extending along a first direction; a gate stack extending across the fin structure along a second direction different from the first direction; and a 2D-material channel layer wrapping around three sides of the fin structure and being wrapped around by a ferroelectric layer of the gate stack when viewed in a cross section taken along the second direction.
 17. The semiconductor device of claim 16, further comprising: a boron nitride layer wrapped around by the 2D-material channel layer when viewed in the cross section taken along the second direction.
 18. The semiconductor device of claim 16, wherein the gate stack further comprises a capping metal layer wrapping around the ferroelectric layer when viewed in the cross section taken along the second direction.
 19. The semiconductor device of claim 18, wherein the gate stack further comprises a polysilicon layer wrapping around the capping metal layer when viewed in the cross section taken along the second direction.
 20. The semiconductor device of claim 16, further comprising: a source/drain metal layer wrapping around the three sides of the fin structure when viewed in another cross section taken along the second direction. 